There are several references in the literature to the use of palladium compounds for the dimerization of acrylate esters. For example, U.S. Pat. No. 4,451,665, issued May 29, 1984, disclosed the use of tetrakis(hydrocarbylnitrile)palladium(II)tetrafluoroborate as a catalyst for the dimerization of acrylate and methacrylate esters. Oehne and Pracejus, in Tetrahydron Letters (1979, pp 343-348) discloses the use of a dimerization catalyst prepared from bis(benzonitrile)palladium(II)dichloride and silver tetrafluoroborate. Alderson et al, Journal of the American Chemical Society (1965) 87 pp 5638-5645 discloses the use of rhodium chloride in an uncomplexed form to dimerize methyl acrylate.
Dialkyl hexenedioates are readily convertible to adipic acid (hexanedioic acid) by hydrogenation and subsequence hydrolysis. Adipic acid in turn is used in large volume in the production of the condensation polymers, particularly Nylon 66. Therefore, even a small improvement in any method for making dialkyl hexenedioates can be of major commercial significance.
A system that will catalyze the dimerization of methacrylate esters to "linear" diesters, that is, 2,5-dimethylhexenedioates, which are the products having the longest possible chain with terminal alkoxycarbonyl groups, is also a desirable objective. 2,5-Dimethylhexanedioic acid, available from dialkyl 2,5-dimethylhexenedioates, is also useful in making condensation polymers such as polyamids and polyesters.